This invention relates to toner monitoring apparatus for an electrographic copier/duplicator having a transparent photoconductor for receiving latent images to be developed by toner particles of a developer mixture. More specifically, the invention relates to the positioning of the toner monitoring apparatus so that a beam of radiation used for monitoring toner concentration passes through the photoconductor and the photoconductor shields the monitoring apparatus from contact with the developer material.
It is well known in the art to provide toner concentration monitoring apparatus which monitors the concentration of toner particles in a developer mixture containing toner and carrier particles. The toner monitor apparatus may comprise a light emitting diode that directs a beam of light in the infrared portion of the spectrum through a window in the housing of the developer station and onto a portion of the developer mixture in the station. Light rays reflected from the mixture pass back through the window and are detected by a detector. The reflectivity of the developer mixture depends upon the relative proportions of toner particles and carrier particles in the mixture. The detector is coupled to a control circuit for a toner replenisher so that additional toner particles are provided to the developer station when concentration of toner in the developer mixture reaches a predetermined level. Apparatus of this general type is disclosed in commonly assigned U.S. Pat. No. 4,141,645 which issued on Feb. 27, 1979, in the names of M. G. Reid et al.
Monitoring systems of the type described above have been satisfactory in operation. However, there are certain problems associated with prior systems. For example, sometimes dusting of developer material on the sensor or airborn dust in the developer station adversely influence the accuracy of the monitoring apparatus. Also, prior systems may monitor the developer mixture by directing electromagnetic rays through a window in a housing of magnetic brush developer station and onto developer nap on a rotating developer roller. The rays may be in the infrared portion of the spectrum and frequently are referred to as "light" rays. The light rays typically are directed at a portion of the nap moving between a developer sump in the housing and the photoconductor. Thus such systems are sensitive to the direction of rotation of nap on the roller. Also, the area of the nap struck by light rays of the monitoring apparatus is alternately rough and smooth due to the relative movement of developer material and magnets used to transport the developer material from the sump to the photoconductor. As a result, the monitoring apparatus varies as a function of the rate of movement of the developer roller (and thus the nap). Moreover, the need to provide a window through which light rays are transmitted to and from the developer roller requires the window to be positively sealed in the housing of the developer station. If the window is not adequately sealed, or if the seal deteriorates after a period of time, toner particles can pass around the window through the leaky seal and contaminate the copier/duplicator. The toner concentration monitoring apparatus of the present invention avoids these problems.